Fuel cell stack with separator of a laminate structure

ABSTRACT

The present invention provides a fuel cell stack including a plurality of unit cells laid one upon another. Each of the unit cells includes an electrolyte, a pair of electrodes that are arranged across the electrolyte and respectively have a catalytic reaction layer, and a separator having means for feeding a supply of gaseous fuel to one of the electrodes and a supply of oxidant gas to the other of the electrodes. The separator is a laminate including a gas-tight conductive plate A and another conductive plate B having at least one slit, which continuously meanders from one end to another end of the conductive plate B. The technique of the present invention gives a compact fuel cell stack assembled by a simple process.

TECHNICAL FIELD

The present invention relates to a fuel cell, especially a polymerelectrolyte fuel cell, which is used for portable power sources,electric vehicle power sources, and domestic cogeneration systems.

BACKGROUND ART

The fuel cells, especially the polymer electrolyte fuel cells, cause afuel gas such as hydrogen, and an oxidant gas such as the air, to besubjected to electrochemical reactions at gas diffusion electrodes,thereby generating the electric power and the heat simultaneously.

The structure of a conventional polymer electrolyte fuel cell isdescribed below.

FIG. 2 is a sectional view schematically illustrating a membraneelectrode assembly (hereinafter referred to as MEA) in the conventionalpolymer electrolyte fuel cell. A pair of catalytic reaction layers 12,which are mainly composed of carbon powder with a platinum catalyst, areclosely attached to both faces of a polymer electrolyte film 11. A pairof diffusion layers 13 having both the gas permeability and theelectrical conductivity are further arranged on the respective outerfaces of the catalytic reaction layers 12. The polymer electrolyte film11, the pair of catalytic reaction layers 12, and the pair of diffusionlayers 13 constitute an MEA 14.

A pair of conductive separator 15 is placed at both face of the MEA 14,so that a plurality of MEAs 14 are electrically connected with oneanother in series. A gas flow path 15 a is formed between the separator15 and the MEA 14 in order to supply fuel such as hydrogen gas andoxidant gas to the electrode, and in order to flow out a gas generatedby the electrochemical reaction and non-reacted remaining fuel gas. Thegas flow path may be provided independently of the separator, but ingeneral, grooves formed on the surface of the separator 15 function asthe gas flow path 15 a. A conventional example of the separator 15 is acut piece of a plate, which is obtained by sintering glassy carbon underhigh pressure at high temperature.

A cooling flow path is provided on the other surface of the separator 15to circulate cooling water and keep the temperature of the fuel cell.Circulation of cooling water enables the thermal energy generated by thereaction to be utilized, for example, in the form of hot water.

Gas sealings and O-rings are placed around the MEA 14 across the polymerelectrolyte film 11, in order to prevent gasses from leaking or frommixing with each other and in order to prevent the cooling water fromleaking. Gaskets that are composed of a resin or a metal plate and havesubstantially the same thickness as that of the MEA may also be arrangedaround the MEA, and the clearance between the gasket and the separatormay be sealed with a grease or an adhesive.

In most cases, a large number of unit cells are laid to construct astack structure of fuel cells. A cooling plate is provided for every oneor two unit cells, in order to cool the fuel cell. The cooling plate isgenerally a thin metal plate in which a cooling water is flowing.Another possible configuration makes the separator itself function as acooling plate. In this case, a water path is formed on the rear face ofthe separator, which includes in each unit cell. In this structure,O-rings and gaskets are also required to seal a cooling water. TheO-rings in the seal should be smashed or flattened completely to ensurethe sufficient electrical conductivity between the cooling plate.

In the stack of fuel cells, the conventional arrangement has an internalmanifold, in which supply inlets and exhaust outlets of gases andcooling water to and from the respective unit cells are disposed insidethe cell. In the case where a reformed gas is used as the fuel gas,however, the CO concentration rises in the downstream area of the flowpath of the fuel gas in each unit cell. This may cause the electrode tobe poisoned with CO, which results in lowering the temperature andthereby further accelerating the poisoning of the electrode. In order torelieve the deterioration of the cell performance, an external manifoldis noted as a preferable configuration that increases the length of thegas supply and exhaust system between the manifold and each unit cell.

In either of the internal or the external manifold, the gas flow pathsshould be formed by a cutting process when a dense carbon plate orglassy carbon plate having the gas tight property is used for thematerial of the separators. The cutting process, however, undesirablyprevents mass production of the fuel cells with a low manufacturingcost.

The carbon plate typically has porosity and thereby relatively poor gastight property. A carbon plate impregnated with a resin is thusgenerally used for the separators of the fuel cells. The cured resin,however, hardly has elasticity, so that the carbon plate impregnatedwith the resin after the cutting process of the gas flow paths may havea warpage. It is thus required to carry out the formation of the gasflow paths after the carbon plate is impregnated with the resin. When aphenol resin or a silicone resin is used as the impregnating agent, theseparator has insufficient acid resistance.

Another possible application mixes carbon powder or metal powder with aresin and manufactures a separator by compression molding or injectionmolding. In this case, the resin should have sufficient acid resistance.When a hard material like polytetrafluoroethylene is used for theseparator, the molded separator does not have sufficient fluidity.

When the resin used as the impregnating agent has poor fluidity, it isrequired to decrease the content of the resin. A specific part of themolded separator that requires the gas tight property should thus beimpregnated again with a resin.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a compact fuel cellstack that is manufactured by a simple process.

At least part of the above and the other related objects is realized bya fuel cell stack including a plurality of unit cells. Each of the unitcells includes a polymer electrolyte film, a pair of electrodes that arearranged across the polymer electrolyte film and respectively have acatalytic reaction layer, and a pair of separators, one having a meansfor supplying fuel gas to one of the electrodes and the other having ameans for supplying oxidant gas to the other of the electrodes. Theseparator has a laminate structure comprising a gas-tight conductiveplate A and another conductive plate B having at least one slit, whichcontinuously meanders from one end to another end of the conductiveplate B.

In accordance with one preferable application of the present invention,the laminate structure includes at least one gas-tight conductive plateA and at least two conductive plates B, and the gas-tight conductiveplate A is disposed on both outer-most layers of the fuel cell stack.

In one preferable embodiment of the present invention, the slit has anend that is not open to outside on a plane of the conductive plate B,and the fuel cell stack has an internal manifold that causes a gasses tobe fed to and discharged from each of the unit cells.

In another preferable embodiment of the present invention, the slit hasan end that is open to outside on a plane of the conductive plate B, andthe fuel cell stack has an external manifold that is arranged on a sideface of the fuel cell stack, which causes gasses to be fed to anddischarged from each of the unit cells.

In still another preferable embodiment of the present invention, theconductive plate B has a lug formed by an end of the slit, and the fuelcell stack has an external manifold that is arranged on a side face ofthe fuel cell stack, which causes gasses to be fed to and dischargedfrom each of the unit cells.

In this structure, it is preferable that the lug is located inside theexternal manifold.

It is preferable that the electrolyte is a proton-conductive polymerelectrolyte.

It is also preferable that the separator has a side face sealed with agas-tight material in the fuel cell stack.

It is further preferable that the separator has a laminating surfacesealed with a gas-tight material in the fuel cell stack.

It is also preferable that the conductive plate B is a punched metalplate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view schematically illustrating an MEA in apolymer electrolyte fuel cell in according with the present invention.

FIG. 2 is a sectional view schematically illustrating an MEA in aconventional polymer electrolyte fuel cell.

FIG. 3 is a perspective view schematically illustrating a conductiveplate that is included in a separator of the present invention and hasat least one slit, which continuously meanders from one end to anotherend of the conductive plate.

FIG. 4 is a perspective view schematically illustrating anotherconductive plate that is included in a separator of the presentinvention and has a slit.

FIG. 5 is a perspective view schematically illustrating an externalmanifold-type fuel cell stack in one embodiment of the presentinvention.

FIG. 6 is a perspective view schematically illustrating a conductiveplate that is included in a separator for the external manifoldconfiguration of the present invention and has a slit.

FIG. 7 is a perspective view schematically illustrating anotherconductive plate that is included in a separator for the externalmanifold configuration of the present invention and has a slit.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a fuel cell stack comprising a pluralityof unit cells, each of the unit cells comprising a polymer electrolytefilm, a pair of electrodes that are arranged across the polymerelectrolyte film and respectively have a catalytic reaction layer, and apair of separators, one having a means for supplying fuel gas to one ofthe electrodes and the other having a means for supplying oxidant gas tothe other of the electrodes, wherein the separator has a laminatestructure comprising a gas-tight conductive plate A and anotherconductive plate B having at least one slit which continuously meandersfrom one end to another end of the conductive plate B.

The present invention is characterized by the specific arrangement thatthe separator has a laminated structure by a gas-tight conductive plateA and another conductive plate B having at least one slit, whichcontinuously meanders from one end to another end of the conductiveplate B. Namely the separator of the present invention mainly comprisestwo different conductive plates. One is the flat conductive plate Ahaving gas tight property, and the other is the conductive plate Bhaving at least one slit that continuously runs in zigzag from one endto the other end of the conductive plate B. In other words, the slit isin the serpentine form.

The conductive plates A and B may be composed of stainless steel, ametal such as aluminum, glassy carbon, or an electrically conductiveresin. The conductive plate A is a flat plate without any pores, holesor grooves and has gas tight property.

In order to constitute a unit cell, a pair of separators, each includingat least one conductive plate A and at least one conductive plate B, arearranged on both sides of the MEA. In particular, a laminate of oneconductive plate A and two conductive plates B arranged across theconductive plate A is disposed between two adjacent MEAs in the unitcells. A laminate including the conductive plate A and the conductiveplate B is arranged on both outer-most layers of the fuel cell stack orthe unit cells.

The following describes the conductive plate B, which is the mostimportant characteristic of the present invention.

The conductive plate B applicable for the internal manifold-type fuelcell stack is described with reference to FIGS. 3 and 4.

FIG. 3 is a perspective view schematically illustrating a conductiveplate B 30 having at least one slit that continuously meanders from oneend to another end of the conductive plate B. The conductive plate Bshown in FIG. 3 functions to make a flow of the gas in the separators ofthe fuel cell stack of the present invention.

Referring to FIG. 3, a conductive plate B 26 has two slits 31 thatcontinuously meander from one longitudinal end to the other longitudinalend of the conductive plate B 26. The slit 31 forms a flow path ofeither the gaseous fuel or the oxidant gas.

The conductive plate B is applied for the internal manifold-type fuelcell stack, and has gas manifold through holes 32 and cooling watermanifold through holes 33 formed therein. Both ends of the slit 31 arelocated on the opposite corners of the conductive plate B and are notopen to outside, so as to form mating gas manifold through holes 34. Inthe case of the external manifold-type fuel cell stack, on the otherhand, the conductive plate B does not have these manifold through holesand has a slit having both ends open to outside as described later.

The slit 31 as shown in FIG. 3 has two bends 35, but the number of bendsmay be changed according to the requirements.

FIG. 4 shows another example of the conductive plate B that makes a flowof cooling water in the separator.

In a conductive plate B 40 shown in FIG. 4, a slit that continuouslymeanders from one end to the other end of the conductive plate B 40 hasends 41, which are located substantially on the center of the ends ofthe conductive plate B 40. The conductive plate B 40 also has gasmanifold through holes 42 and 43 and cooling water manifold throughholes at the ends 41.

The slit has six bends 44 to improve the cooling ability in the exampleof FIG. 4, but the number of bends may be changed according to therequirements.

With reference to FIGS. 6 and 7, the following describes the conductiveplate B applicable for the external manifold-type fuel cell stack asshown in FIG. 5.

A conductive plate B 60 shown in FIG. 6 has a slit 60 that continuouslymeanders from one end to the other end of the conductive plate B. Unlikethe conductive plates B shown in FIGS. 3 and 4, the conductive plate Bshown in FIG. 6 does not have manifold through holes for flows of thegas and the cooling water.

In the conductive plate B shown in FIG. 6, the slit 60 is open tooutside of the conductive plate B and is constituted by two members 62 aand 62 b. The two plates 62 a and 62 b punched out in a comb-like shapeare combined together to form the slit 60.

A conductive plate B 72 shown in FIG. 7 solves the problem of theconductive plate B shown in FIG. 6, that is, the separation of theconductive plate B into two members. A slit 70 has one end 71, whichforms a lug 71 located outside the conductive plate B 72 and is not opento the outside. This arrangement prevents the conductive plate B frombeing separated into two members and facilitates the assembly of thefuel cell stack.

In the external manifold-type fuel cell stack, it is preferable thatexternal manifolds for feeding and discharging the gases to and from therespective unit cells are disposed on the side faces of the fuel cellstack. In the case of the conductive plate B 72 shown in FIG. 7, theends 71 of the slit 70 are preferably located inside the externalmanifolds. This desirably makes the fuel cell stack compact.

The conductive plate B included in the separator of the presentinvention is punched out of the metal plate discussed above.

It is preferable that the separator laminate including the conductiveplate B has a side face and/or a laminating face sealed with a gas-tightmaterial.

Some examples of the present invention are given below, although thepresent invention is not restricted at all to these examples.

EXAMPLE 1

A carbon powder having the particle diameter of not greater than severalmicrons was dipped into an aqueous solution of chloroplatinic acid. Theplatinum particle was carried on the surface of the carbon powder byreduction of the aqueous solution. The weight ratio of carbon toplatinum carried thereon was 1:1. Then, the carbon powder with theplatinum catalyst carried thereon was dispersed into an alcohol solutionof a polymer electrolyte to obtain a slurry.

On the other hand, a carbon paper having a thickness of 400 μm, whichwas the supporting material of electrodes, was impregnated with anaqueous dispersion of a fluorocarbon resin (Neoflon ND-1 manufactured byDAIKIN INDUSTRIES, LTD.). After drying, the impregnated carbon paper washeated at 400° C. for 30 minutes to give the water repellency to thecarbon paper.

Referring to FIG. 1, the slurry containing the carbon powder was applieduniformly on a single face of water-repelled carbon paper 21 to form acatalytic layer 22 of 20 μm in thickness and obtain an electrode 23.

A pair of the carbon-paper electrodes 23 having the catalytic layers 22was laid across a polymer electrolyte film 24 of 25 μm in thickness insuch a manner that the respective catalytic layers 22 of the electrodes23 were in contact with the polymer electrolyte film 24, followed bydrying to obtain an MEA 25. The polymer electrolyte film 24 was aproton-conductive polymer electrolyte comprising of a polymer ofperfluorocarbon sulfonate. A pair of conductive plates B 26, whichconstituted the separators, was arranged across the MEA 25 and a pair ofconductive plates A 27, which constituted the separators, was arrangedon both outer faces of the conductive plates B 26. This completed a unitcell.

The conductive plate B 26 comprised SUS 316 having a thickness of 1 mm.Slits 31 having a width of 2 mm for the flow of a gas were formed bymeans of laser on the surface of the conductive plate B 26 as shown inFIG. 3. The conductive plate B 26 also had gas manifold through holes 32and cooling water manifold through holes 33 disposed in the peripherythereof. In the process of interposing the MEA 25 between the pair ofconductive plates B 26, gaskets having the same outer dimensions asthose of the conductive plates B 26 were disposed around the electrodes23. The gaskets were obtained by bonding a pair of sheet of anethylene-propylene-diene terpolymer compound to both surfaces of a sheetof polyethylene terephthalate sheet.

After lamination of two such unit cells, a pair of the conductive platesB 40 having the slit for making the cooling water flow path was disposedas shown in FIG. 4, across the laminated unit cells. This completed aunit cell laminate.

A fuel cell stack was assembled by laying fifty unit cell laminates,each including two unit cells, one upon another and disposing a pair ofmetallic current collectors, a pair of insulator plates, and a pair ofend plates in this order on both sides of the cell laminate. The endplates were fastened and fixed with bolts passing through the celllaminate and nuts. This completed a fuel cell stack including 100 unitcells.

Evaluation

A battery test was carried out while the gaseous hydrogen and the airwere flown into and the cooling water was circulated through the fuelcell stack. The cell output power was 1020 W (30 A-35 V) under theconditions of the hydrogen utilization of 70%, the oxygen utilization of20%, the humidified hydrogen bubbler temperature of 85° C., thehumidified oxygen bubbler temperature of 75° C., and the celltemperature of 75° C.

As described above, each separator of Example 1, which parts theadjoining unit cells from each other, includes the gas-tight conductiveplate A and the conductive plate B having the grooves for the flow of afluid. Compared with the conventional carbon separators, thisarrangement advantageously reduces the thickness of the separators andthereby decreases the manufacturing cost.

EXAMPLE 2

In Example 2, the side faces of a fuel cell stack, which included 100unit cells and was manufactured in the same manner as in Example 1, wassealed with a gas-tight rubber. The gas-tight rubber used here was aphenol resin. The phenol resin solution was spreaded on the side facesof the fuel cell stack and dried to seal the side faces of the fuel cellstack.

Evaluation

A battery test of the fuel cell stack thus obtained was carried outunder the same conditions as those of Example 1. The output power of thefuel cell stack was 1080 W (30 A-36 V).

A leak test of this fuel cell stack was also carried out. In the test,an outlet of cooling water was closed and a hydraulic pressure wasapplied from an inlet of cooling water. No leakage of water was observedunder the hydraulic pressure of 1 kgf/cm². This proved the sufficientsealing property. From the result, it was confirmed that the arrangementof sealing the side faces of the fuel cell stack is extremely effectiveto improve the sealing property of the fuel cell stack.

The method of Example 2 applies the sealing rubber on the side faces ofthe fuel cell stack, which has been obtained previously by laying aplurality of unit cells and fixing the cell stack with the end plates.This method significantly decreases the number of manufacturing steps,compared with the conventional method that spreads the sealing rubber onthe side faces of each unit cell while the unit cells are laid one uponanother.

EXAMPLE 3

In Example 3, an external manifold-type fuel cell stack was assembled asshown in FIG. 5 from the unit cells, which were manufactured in the samemanner as in Example 1.

In first, specific parts corresponding to the internal manifolds werecut out from the unit cells manufactured in Example 1, and caused inletsand outlets of gases and cooling water to be exposed to the side facesof the unit cells.

A phenol resin used as the sealing rubber was spreaded on the side facesof the fuel cell stack and dried to seal the side faces of the fuel cellstack. In this example, the path of the inlets and outlets of gases andcooling water were not sealed by the sealing rubber. The phenol resinsolution was carefully spreaded to make a flat sealing surface, whichwas in contact with the external manifold.

As shown in FIG. 5, the external manifolds 51 of stainless steel wasarranged on the side faces of a unit cells 8 to cover an array of theexposed inlets of the air. In a similar manner, external manifolds werearranged to cover the outlets of the air, the inlets and outlets ofhydrogen gas, and the inlet and outlets of the cooling water. Theseexternal manifolds were fixed with screws for the end plates.

The gaskets were obtained by cutting a sheet of ethylene-propylene-dieneterpolymer compound having closed cells to a predetermined shapecorresponding to the sealing surface of the external manifold. Thegaskets were used for sealing the clearance between the externalmanifold and the sealing material covering the side face of the celllaminate.

Evaluation

A battery test of the fuel cell stack thus obtained was carried outunder the same conditions as those of Example 1. The output power of thecell stack was 1080 W (30 A-36 V). The seal of the external manifold waschecked for a gas leak. No leakage was observed, which proved thefavorable sealing property.

As described above, the arrangement of spreading the sealing rubber onthe whole side face of the polymer electrolyte fuel cell enables theexternal manifold configuration, which is typically applied for themolten carbonate fuel cells.

The arrangement of Example 3 enables the manifolds and the cell to bemanufactured separately. This enables the unit cells having theidentical shape of separators and MEAs to be mass production, regardlessof the applications and the output power. This advantageously reducesthe manufacturing cost.

EXAMPLE 4

The process of Example 4 applied the conductive plate B shown in FIG. 6for the separator having the flow paths of gas and cooling water andassembled an external manifold-type fuel cell stack by laying the unitcells manufactured in Example 3.

Evaluation

A battery test of the fuel cell stack thus obtained was carried outunder the same conditions as those of Example 1. The output power of thecell stack was 1080 W (30 A-36 V).

The conductive plate B used for the separator has a through groove inthe thickness of the conductive plate B. The through slit continuouslymeandering from one end to the other end of the conductive plate Bfavorably reduces the manufacturing cost.

EXAMPLE 5

The process of Example 5 applied the conductive plate B shown in FIG. 7for the separator having the flow paths of gas and cooling water andassembled an external manifold-type fuel cell stack by laying the unitcells manufactured in Example 4.

Evaluation

A battery test of the fuel cell stack was carried out under the sameconditions as those of Example 1. The output power of the cell stack was1080 W (30 A-36 V).

The conductive plate B of Example 4 was fabricated by two separate partsand thus requires some time for assembly. The conductive plate B ofExample 5, however, has only one part and thus improves the assemblingproperty. In the separator of Example 5, the ends of the slit extendoutside the contour of the conductive plate B. This configuration,however, did not prevent the external manifolds from being attached tothe cell without any troubles.

The conductive plates B having the shape shown in FIG. 7 weremanufactured by etching and pressing. These plates gave substantiallythe same results in the battery test.

INDUSTRIAL APPLICABILITY

As described above, the technique of the present invention simplifiesthe assembly of the fuel cell stack and provides a fuel cell stackhaving the improved gas sealing property between the external manifoldsand the side faces of the cell stack. The technique of the presentinvention has especial advantage for a stack of polymer electrolyte fuelcells.

What is claimed is:
 1. A fuel cell stack comprising a plurality of unitcells, each of said unit cells comprising; an assembly comprising apolymer electrolyte film and a pair of electrodes that are arrangedacross said polymer electrolyte film and respectively have a catalyticreaction layer; and a pair of separators that are arranged across saidassembly, one having a means for supplying fuel gas to one of saidelectrodes and the other having a means for supplying oxidant gas to theother of said electrodes, wherein said separators between saidassemblies of adjacent unit cells are formed as a laminate of at leastone gas-tight conductive metal plate A, and at least two conductivemetal plates B each having at least one slit which continuously meandersfrom one end to another end of said conductive plate B, each of saidconductive metal plates B being a punched metal plate, wherein theoutermost separators of said fuel cell stack are each formed as alaminate of another gas-tight conductive metal plate A and anotherconductive metal plate B having at least one slit, which continuouslymeanders from one end to another end of said conductive plate B, saidanother conductive plate B being a punched metal plate, wherein eachconductive plate B has a lug formed by an end of said slit, said fuelcell stack furthering comprising an external manifold that is arrangedon a side face of said fuel cell stack, so as to cause a gas to be fedto and discharged from each of said unit cells.
 2. The fuel cell stackin accordance with claim 1, wherein said lug is located inside saidexternal manifold.
 3. The fuel cell stack in accordance with claim 1,wherein said electrolyte is a proton-conductive polymer electrolyte. 4.The fuel cell stack in accordance with claim 1, wherein each separatorhas a side face sealed with a gas-tight material.
 5. The fuel cell stackin accordance with claim 1, wherein each separator has a laminatingsurface sealed with a gas-tight material.